Thermoforming is a two step process of heating a thermoplastic material to a workable temperature, typically sightly below the material's melting point, and then vacuum or pressure forming the material into the desired shape through one of several techniques. Thermoforming processes are used to produce deli and drinking cups, storage containers and the like from extruded polypropylene, propylene/ethylene copolymer and other materials. Various thermoforming techniques are described in the 1989 mid-October issue of Modern Plastics Encyclopedia, pp. 301-306.
During the heating step it is highly desirable to reach a uniform temperature distribution across the sheet of thermoplastic material in order to achieve a more uniform distribution of material in the part to be produced. Heating the thermoplastic material, however, tends to cause the material to sag under the influence of gravity; this tendency is known as thermoforming sag. The extent of thermoforming sag is an important parameter as it controls the "window of operation" for the thermoforming process. In other words, the tendency of the thermoplastic material to sag when heated limits the time available to achieve a uniform temperature distribution across the sheet and thus limits the time/temperature window of operation.
Typical polypropylene and propylene/ethylene copolymers used in thermoforming processes have a general tendency to sag when heated. Thus there is a need for polypropylene and propylene/ethylene copolymers suitable for use in thermoforming processes that have an increased resistance to thermoforming sag.
Boynton, U.S. Pat. No. 4,282,076 discloses a method of visbreaking polypropylene wherein a prodegradant is formed by activating a first portion of a polypropylene polymer, mixing the prodegradant with a second portion of propylene polymer which may contain a stabilizing amount of at least one antioxidant, wherein the second portion is at least equal in amount to the prodegradant, adding to the mixture of the prodegradant and second portion of the propylene polymer a stabilizing amount of at least one antioxidant if the second portion does not already contain a stabilizing amount of such a stabilizer, and heating the mixture to an extrusion temperature to controllably lower the molecular weight of the mixture, while substantially retaining the stabilizing effect of the antioxidant stabilizer or stabilizers. The prodegradant is produced by activating a portion of the polypropylene polymer by exposure to ionizing radiation or air oxidation.
Kohyama et al., U.S. Pat. No. 4,727,113, discloses a crystalline 1-butene comprising: (a) a crystalline 1-butene polymer containing a 1-butene component as a main component, and (b) a radical-treated crystalline olefinic polymer having (1) a boiling p-xylene insoluble content of 30% by weight at most and (2) the difference between the crystallization temperature of the radical-treated crystalline olefinic polymer and the crystallization temperature of the crystalline olefinic polymer before the radical treatment being greater than or equal to 1, and (c) the proportion of the radical-treated crystalline olefinic polymer (b) being 0.2 to 100 parts by weight of the crystalline 1-butene polymer. The radical treatment purportedly may be carried out by treating the crystalline olefinic polymer in the molten state by the action of a shearing force in the presence of a cross-linking agent and a polymerization initiator, or exposing the crystalline olefinic polymer to light irradiation or ionizing irradiation in the presence of a photo-polymerization initiator.
Kirch, German Application DE 3,415,063, discloses a process for nucleation of partially crystalline plastics by irradiation wherein neutron beams are applied. Purportedly, the neutron beams, because of their different physical mode of action, as compared to electron, X-ray, gamma or ultraviolet beams, interact primarily with hydrogen atoms which reduces the number of chain breaks. Moreover, Kirch states that treatment with electron, X-ray, gamma or ultraviolet beams cause an undesired intensive degradation which alters the properties of the starting polymers. Kirch discloses neutron emitters such as americium-241/beryllium, californium-252, spent nuclear fuel rods and neutron radiation occurring in the operation of nuclear reactors as irradiation sources.
Fisher, U.S. Pat. No. 4,628,073 discloses a soft-rubbery matrix material, and a method of producing the material, wherein the material is composed of 0.3-70 micron particles of a 50,000-300,000 molecular weight cross-linkable polymer dispersed in a fluxable elastomer where the polymer's softening point temperature exceeds the elastomer's fluxing temperature and the polymer and elastomer are combined and mixed at a temperature maintained above the fluxing temperature of the elastomer but below the softening point temperature of the polymer. When a cross-linked polymer component is desired, high-energy ionizing radiation induced cross-linking is the preferred practice.
Potts, U.S. Pat. No. 3,349,018, discloses a method for controllably degrading alpha olefin polymers such as polypropylene without the use of heat and/or mechanical shear. In the method of Potts, polypropylene is degraded by subjecting it to ionizing radiation to a total dose of between about 0.01 to about 3 megareps but below that amount which causes gelation. The results of the method of Potts are attributed to uniform treatment of every portion of the resin mass by high energy ionizing radiation and it is stated that in the process each molecule of resin is surrounded by a cloud of high energy particles so that no portion of the polymer is able to escape treatment. Additionally, in a preferred embodiment of Potts, a small amount of antioxidant, preferably about 0.01 to about 0.1 percent by weight of anti-oxidant is incorporated prior to subjecting the polymer to ionizing irradiation.
Scheve, U. S. Pat. No. 4,916,198, discloses a purportedly normally solid, high molecular weight, gel-free, amorphous to predominantly crystalline, propylene polymer characterized by high melt strength due to strain hardening believed to be caused by free-end long chain branches of the molecular chains forming the polymer. The material is characterized by a branching index preferably less than 0.9 and most preferably about 0.2-0.4. Scheve also discloses a process for making the polymer by high energy radiation of a normally solid, high molecular weight, linear polypropylene polymer in a reduced oxygen environment, maintaining the irradiated material in such environment for a specific period of time, and then deactivating free radicals in the material.
There remains, however, a need for a polypropylene or propylene/ethylene copolymer with improved thermmoforming sag resistance. Preferably, the polypropylene or propylene/ethylene copolymer with improved thermoforming sag is practically and readily obtained to provide the advantages of improved thermmoforming sag resistance.